The present invention relates to methods for forming precision ball race groove tracks in a part, especially in the outer race member of a universal joint.
There are many mechanical parts having ball race groove tracks formed therein for engagement with bearing balls. The ball race groove tracks may be produced by various methods including formation of the ball race groove track with the part when the part is forged or cast, grinding, machining or cold forming the track into a rough workpiece, or cold forming or forging an initial roughly proportioned track during the formation process for the part and subsequently grinding or machining the formed ball track into a precision shape. The last mentioned methods are the most common when a precision shape for the ball race groove track is required since an accurately dimensioned ball race groove is produced without a substantial waste of material. Furthermore, these methods of producing a ball race groove cause less wear on the grinding machining tool than is the case when the ball race groove track is ground in a workpiece not having an initially formed ball race groove track therein.
Precision ball race groove tracks are required, for example, for universal joints of the type having spherically engaged inner an outer race members coupled to each other by a plurality of bearing balls disposed in ball engaging meridian race grooves in the inner and outer race members. Examples of this type of universal joint may be found in U.S. Pat. No. 2,046,584, issued July 7, 1936 to Alfred H. Rzeppa and in U.S. Pat. No. 1,665,280, issued Apr. 10, 1928 to the same inventor. A significant advantage of this type of construction for a universal joint is that it has a constant velocity property. That is, the speed of rotation of the shaft interconnected with the inner race member is the same as that of the shaft interconnected with the outer race member, regardless of the relative angular position between the inner and outer race members within a predetermined range of relatived angular positions.
For this reason, this type of universal joint has become popular for use in front wheel drive assemblies for motor vehicles.
The typical constant velocity universal joint of the above described type, often referred to as a Rzeppa joint, requires six precision meridian ball tracks in a spherical cavity in an outer race member as well as six precision meridian ball tracks formed in the outer spherical surface of an inner race member. These tracks are initially formed in the inner and outer race members when the parts are forged or cast and are subsequently finish machined to produce a desired precision shape for each of the ball tracks. During the grinding or finish machining operation, a portion of the working surface of the tool is worn, thus, requiring periodic replacement of the tool in order to maintain the accuracy of the finish machining or grinding operation. Such periodic replacement is expensive in terms of the cost of replacing the tool and in terms of the loss of operating time while the tool is being replaced.
When a ball race groove track is formed in such a part by a cold forming operation, the metal of the part is substantially displaced, particularly in the region of the apex of the ball track. The metal in the region of the apex of the ball race groove track offers the greatest resistance to the formation of the ball race groove track. Thus, some relief in the region of the apex of the ball race groove track would be advantageous in order to facilitate the formation process.
When a ball race groove track is to be ground in such parts, a grinding tool is rotated about an axis extending outwardly from the apx of the ball track. Thus, the portion of the grinding tool in the vicinity of the apex of the ball race groove track is moving comparatively slowly and encounters the greatest amount of friction. The friction in the region of the apex of the ball track causes rapid deterioration of the grinding tool, thereby requiring frequent replacement of the grinding tool and a large amount of down time. This friction also creates heat which may generate cracks in the part. Thus, each part must be carefully inspected for microscopic cracks. A noticeable amount of scrap results from the detection of such microscopic cracks during such inspections, increasing the manufacturing costs per part. Quality control problems may also be encountered as a result of undetected flaws.
Accordingly, what is needed is a method for producing ball race groove tracks in Rzeppa joints and similar constant velocity universal joints, as well as other machine elements requiring precision ball race groove tracks, which method reduces the amount of wear on the grinding or finish machining tool, and the amount of scrap resulting during the manufacture of such ball race groove tracks.